Add to that wave power, custom built ships just for this purpose anchored in place, fiber connection to the mainland and it may well prove to be cheaper over the long term than a land-based air conditioned building that requires lots of power. Air conditioning is a huge part of the long term cost of a datacenter, using water cooling with abundant supplies of water seems like a very green way of doing things.

Computers can go in a matter of months in a location really close to the sea.

On the other hand, I know people, in the town I have just moved to, who live only tens of meters from the sea who have had no problems - but they have a massive rampart between them and the sea that (I think) blocks the spray.

Ships are going to be tricky but designs meant to keep salt spray out may be workable.

Not to mention that there's no property tax (being taxed to occupy real estate), if the local business or economic climate goes bad you can pick up and be towed to a different location, and you can always add more units if demand increases.
The one problem I see is pirates. No, seriously - you anchor one of these away from an area patrolled by a decent navy/coast guard, and I can see someone paying you a visit late one night to haul away equipment...

Google datacenters are pretty much disposable today. Build it once, run it for X years, then dump the entire thing. Repairs are less and less useful.

Each rack could be an independently sealed bubble (airtight) with a few wires coming out the top for power and network connectivity, then hang the entire rack into a flooded compartment of the boat -- say a catamaran with a protective mesh bottom.

First, sea water temperatures vary greatly depending on the part of the world you're operating in. It's not uncommon for surface sea water temps to be in the 85F(30C)+ range for most areas where you're likely to moor a ship. The AC units that we used were barely able to keep the small server room that I ran cool under those conditions.

Second, the motion of the ship caused premature drive failures due to the pitch and roll of the ship. This could be alleviated with solid state drives, but that's a bit off for a data center at the moment.

Lastly, bandwidth and latency are problematic. Sure, Google could just buy a satellite, but they can't modify the 2000ms latency. Depending on ship size and sea conditions, keeping a satellite lock may be an issue as well due to roll.

Also, powering the data center using tidal power would be taking energy out of the ocean. While the water cooling would be dumping energy back into the ocean, it will be dumping in less energy than was taken out, so there should actually be a net cooling of the ocean.

Let's do the math. Let's say Google buys the Queen Mary. 80,000 tons. Let's say they anchor it someplace with an average wave height of 20 feet, wave period of 10 seconds. Raising 80,000 tons at 2 feet per second takes about 160,000 horsepower. Hmmm, that's very close to the original steaming capacity of the QM. In watts, that's about 120 megawatts, about ten times more than you'd need if you packed the ship with servers. Okay, so that looks easily doable.

Problem is, buying the electricity would be much cheaper. 12 megawatts will cost you about $700 an hour. Can you run and maintain and pay on the principal and pay salaries and insurance on $700/hour? No, not a couple of powers of ten.